In a cellular phone network, fixed wireless access (FWA) using a millimeter-wave band has been widely utilized. The communication quality of each wireless link is affected by radio environments such as a signal-to-noise ratio (SNR) and a carrier-to-interference-plus-noise ratio (CINR) of received signals. In this regard, adaptive modulation has been attracting attention in order to achieve a wider bandwidth of the wireless link. The adaptive modulation is a technique of adaptively searching and using a modulation method with the highest transmission efficiency depending on the radio conditions of the wireless link. The adaptive modulation technique can provide an optimum wireless communication depending on the radio environments and can improve the frequency efficiency.
In such a network, however, when a path control (for example, Non Patent Literature 1) as used in a wired network is applied, the network is affected by a variation in the transmission rate of each wireless link due to adaptive modulation, which affects the communication quality of traffic. In other words, a path through which traffic flows includes a plurality of wireless links, and the wireless links independently select a transmission rate by adaptive modulation. Accordingly, if a path including wireless links with different transmission rates is selected, the stability of the path itself deteriorates. In particular, since wireless links greatly depend on radio wave environments, it is considered that wireless links have a correlation with each other. Therefore, it is important to take into consideration a correlation of communication quality in a path control operation.
In a typical network using a plurality of wireless links, a normal path and a redundant path different from the normal path are set so as to maintain the communication quality. The redundant path is an extra path which is used in place of the normal path in case of occurrence of a failure, for example. Therefore, even in the case of setting the redundant path, it is necessary to take into consideration a correlation between wireless links so as to prevent the normal path and the redundant path from deteriorating at the same time.
Patent Literatures 1 and 2 propose a method for calculating a physical distance between wireless links as a spatial correlation coefficient and selecting a path with a small spatial correlation coefficient. However, these patent literatures focus only on the physical distance. Thus, there is a possibility that wireless links which are spatially close to each other and have no correlation of communication quality between the wireless links cannot be selected for the redundant path. There is another possibility that wireless links which are spatially distant from each other and have a high correlation of communication quality between the wireless links can be selected for the redundant path. Therefore, such a method is inadequate.